Gain control system responsive to an average value



NOV. 16, 1948. H E, HAYNES I 2,454,169

GAIN CONTROL sYsTEM RESPONSIVE To AN AVERAGE VALUE y Filed Dec. 2e, 1946 INVENTOR AT`roRNEY Patented Nov. 16, 1948 GAIN CONTROL SYSTEM RESPONSIVE TO AN AVERAGE VALUE Harold E. Haynes, Audubon, N. J., assignor to Radio `Corporation of America, a corporation of Delaware Application December 26, 1946, Serial No. '118,376

6 Claims.

The present invention relates to gain control systems and more particularly, but not necessarily exclusively, to a novel gain control system which is responsive to an average value of a characteristic such, for example, as the average of the light intensity impinging on a light sensitive electronic device, such as an electron multiplier phototube and the like, whereby to vary amplification.

In a system operating in accordance with this invention, a very small voltage change controls the high voltage output of a power supply; In combination with an electron multiplier phototube, the controllable power supply of this invention serves in a novel manner as a sensitive automatic gain control. In general, the invention provides a way to use an electron multiplier phototube to derive an alternating current voltage from an amplitude modulated light beam in any application where the average amount of light is nominally constant. The electron multiplier vphototube possesses the highly desirable attributes of great sensitivity and low-noise level. In

kaccordance with the invention the effects of allV sources of gain variation in the system are minimized. Systems in which the average amount of light is nominally constant include, for example, light-beam communication systems, light-beam type phonograph pick-ups and reproduction of sound tracks on for example, not having' ground-noise reduction. Solely, by way of example, a system for reproducing sound` from the sound track of a film has been selected as an illustrative embodiment of the invention `which may be readily understood and is convenient to describe. Where the small control voltage is -derived as a result of reproducing a sound track on a transparent film, for example, a definite response level of the sound reproducing system to average values of light output Afrom the optical system, including the interposed lm, may be maintained. For example, in the embodiment selected for illustrating the controllable power supply of the invention combined with an electron multiplier phototube, an average value of light when directed in an appropriate manner `through or upon a iilm bearing a sound-track will produce an average output current in the phototube upon which the light is directed.

It will be seen from the foregoing that the present invention deals with the combination of a controllable power supply and an electron multiplier phototube. 'I'he power supply selected for the purpose of illustrating the invention herein is disclosed and claimed in a copending applica- (Cl. Z50-41.5)

2 tion of the present inventor for Letters Patent for Power supply for a light sensitive electronic device, filed December 26, 1946, Serial No. 718,375.

An important object of the present invention is to provide a novel signal reproducing system in which the system gain is controlled by the average signal value.

Another object is to provide a novel means for deriving a control voltage which is applied, effectively, to an electrode or electrodes of` an oscillator tube thereby to vary the alternatin current output voltage of the oscillator.

A further object of the invention is to provide a power supply of the type comprising an oscillator for generating a relatively high frequency current and a rectifier for rectifying the generated oscillations, combined with a novel means 1 obtained from the oscillator.`

a sound reproducing system.

Still another object is to provide a novel gain control system embodying an electron multiplier phototube in combination with a controllable power supply for the phototube.

Other objects and advantages of the present invention will, of course, become apparent and immediately suggest themselves to those skilled in the art to which the invention is directed from a reading of the following specification in connection with the accompanying drawings in which:

Fig. 1 is a schematic showing of apower supply system suitable for use in apparatus arranged in accordance with the invention;

Fig. 2 isa schematic showing, by way of example, of a sound reproducing system to be used in combination with the power supply of Fig. 1 to obtain automatic gain control;

Fig. 3 shows a fragment of a motion picture lm having a sound track; and n Fig. 4 is a schematic showing of a typical optical arrangement.

Referring to Fig. 1 of the drawings there is shown a tube Ill which functions as an oscillator. It comprises a cathode I2, a control grid I4, a screen grid I6 and an anode I8. The anode and screen electrodes are `maintained at a positive direct current voltage Vfrom a direct current source (not shown) which may be connected at the terminal indicated schematically at 2 I. The` conments. high D. C. Voltage which is available from the trol grid I4 is returned to the cathode l2 through a path including a grid resistor 22 shunted by a capacitance 23 and a feed-back or tickler coil 24. The latter is tuned by a variable capacitor 2B. The cathode l2 of the tube lil is connected effectively to ground through the space discharge path of a tube 2S which comprises an anode 29 and a cathode-3| in parallel with the capacitor |51. The function and operation of the tube 28 will be described more in detail hereinafter. The anode circuit of the tube Il] includes a parallel resonant circuit which comprises the primary winding -32 of an output transformer 33. The primary winding 32 is tuned by a capacitance 34. The `tickler coil 24, mentioned above, may be onefof the vwindings of the transformer 33.

The secondary 36 of the transformer 33 .is connected to a load resistor 38 through a diode rectifier 39. load resistor 3B to bypass the high frequency cur- 'Ihe symbol VEn represents the-relatively high voltagepower supply source of Fig. 1.

The oscillator comprising the tube was -well Vfas:thetransformer33 areshown by-way of eX- ample. V.Oscillating circuits for the tube lll other than thoseshown-and different transformer ar rangements may be employed since details of gthese elements of :the system are shown only by way of example. .coils.2.4,:32 and 35 are coupled magnetically and .that ,this-coupling with respect to one or all of l It will be understood l that the the coils may bemade-adjustable if desired.

The spacedischarge path, and hence, thein- .ternal impedanceof thetuoe 28, is inseries with the `anode ,circuit of the oscillatortube .l El to vary the. `effective .plate supply `voltage between its -cathode l2andits anode I8. The internal-impedance-of .the tube 28 to valternating current is equalto-itsfD. C. resistance, and would cause a large loss of high frequency power if the A. C.

component y:of yoscillator plate current flowed through it. Therefore, a capacitor IGT yis connectedin parallel withthe internal v.impedance of the tube 28 to provide a low impedance to high frequency current. The tube -28 preferably has .,al-highgridV-plate transconductance. By way of ekamplaa type -GAG'Y .tube is suitable. The plate voltage supply source (not-shown) for the oscilflatortube l will have its negative terminal-re- ,turnedto ground, the connection being indicated schematically and designated by referencecharacter 44. K The screen grid 46 of the tube 28isconnected ,to an appropriatepoint on a voltage divider-comireference characters .52 and .53, respectively.

A considerable degree Hof `control of zthe developed D. C. voltage Eo may be obtained by varying the voltage on the signal grid i of the tube 28. Throughonta considerable portion of the characteristic, the control curve is linear and is suitable for gain control in apparatus employing avphotosensitive device such as an electron .multiplier .phototube `In applicants copending application above referred to identified by Serial iNo. 718,375 asystem'isdisclosed and claimed in which the eifectivegain in a facsimile-system is controlled upon scanningof the white background ofsubject copy in a facsimile scanner. .The presen't invention is directed to the use of .thepower A capacitance 42 is shuntedacross the vwhere there is an average light output inl'the vvice 66 including suitable lenses.

supply of Fig. 1 which, as stated above, 'is disclosed and claimed in the application just mentioned, in combination with a system in which the average amount of light is nominally constant tc obtain gain control in a novel manner.

Fig. 2 of the drawings is illustrative by way of Vexample only ofthe electrical features of such a system embodying the present invention. Fig. 4 is a schematic showing of a typical optical arrangement which may be used to excite a phototube 59 which is or may be a part of any well known optical sound reproducing device or sound head, as it is called. A more or less detailed description of a system suitable for theatre use is described in the RCA Review f-or January, 1937, publishediby the RCA Institutes Technical Press, '75 Varick Street, New York, beginning on page of that publication. Fig. 3 of the drawings shows Va `fragment of a motion picture film Gl, or the like, which is printed with a sound track The inventionis, aspreviously stated, useful optical system which. is impingedl on aphototube.

Iffthe `sound track 62is ofthe class Avtype,and Adoes not have the groundnoise reduction-feature,

described :in .fthe publication .presently `referred to, the `average light output of theoptcal-system includingthe lm is constant.` The transparent part 63 of the sound track is not reduced vin area duringperiods ofsilence when the ground noise reduction feature isomitted. This type ofsound trackfaswell as othertypes which do anddo not have the vground 4noise reduction feature rare shown in the RCAReview, publishedrv bythe rRCA Institutes Technical Press, ,Varick Street,.l\ lew York, for October, 1936 on ypage 14 thereof. It will be understood .that the film, Wliichis fragmentarilyshown in Fig. 3, mayhave any desired form and, infact, the sound track may be printed upon a -narrow transparent strip for use `as a phonograph record or it may .be printed on an 4opaque materialfor use with areflective optical system.

The phototube 59 generates electrical signals in a known manner which yare used after amplication tooperate a loud speaker (not shown).` A sound system of `this. type is shown more in detail inFig. 4 of U. S. Patent #1,888,724, grantedto C. R..:Hanna-on November 22, 1932. The lm 6I is moved at constantspeed by any suitable means (not shown) and light from an appropriate source .such as an exciter lamp v64 after passing through a narrow slit (not. shown separately) is focused on the sound track 62 of the` lml I by an optical de- A collector lens 68 directs light receivedfrorn the sound trackof the lmonto the cathode l'l of the phototube 59. Thephototube .59 is preferably of the electron multiplier type and may, for example, be the commercially available type 931-A multiplier phototube which is of the electrostatically. focused type. The photo-cathode/'Hof the phototube 5,9 (shown schematically inFig. 2) is'connected, for thepurposes of this aspect of the invention, to the negativeterminal of .the high voltage supply oflig. 1 which is at the negative lend of the load resistor 38. For the type of phototube mentioned by way of example, the voltage acrossy the terminals ofl the-power vsupply source is `to be Yapproximately 700 volts.

Automatic gain control in vaccordance with the vinvention is accomplished by varying Athe total '5 dynodes 12 are connected to spaced points along a voltage divider 10 made up of the resistors 13.

VTheA anode 14 of the phototube is connected in theaverage anode current. The resistor 18 is vincluded in'. the connection from the anode 14 to the positive terminal of the load resistor 38 of the power supply of Fig. 1. A D. C. source 8|, shown for the sake of convenience as a battery, provides a positive voltage Ei which is to be opposed by the drop in the resistor 18 so that it is almost bucked out by -this drop. The output from the phototube is taken from the anode 14 through a suitable coupling condenser 83 to the reproducing system (not shown). 'I'he output terminal is indicated schematically at 86.

When the sound reproducing arrangement of Fig. 2 is in operation, and the light intensity the exciter lamp 64, for example, increases, the control voltage E3 would go slightly more negative than before and since this voltage is applied to the control grid 5l of the control tube 28, the high voltage output appearing across the load resistor 38 will be reduced. This reduction of voltage of the high voltage supply and the consequent reduction in gain will continue until an equilibrium point is reached.

The resistor 11 may be made relatively small so that frequency response is as good as required. The resistor 18 may be considerably larger so that a suitably large control voltage E3 may be obtained. For example, assuming that 50 microamperes is a suitable average current and that 20 Volts appearing across the resistor 18 is suitable, the resistor 18 should be about 400,000 ohms. For suitable frequency response, the resistor 11 might be, perhaps, 100,000 ohms. The voltage E1 would then be set at about +21 volts.

Having now described the invention, what is claimed and desired to be secured by Letters Patent is the following:

1. A signal reproducing system comprising means for generating signal currents in response to an incident modulated light beam, said means including an electron multiplier phototube, an output circuit for said phototube comprising a pair of resistors in series, means for obtaining a control voltage from the junction point of said resistors, means for utilizing said control voltage to control said phototube means for obtaining a signal output across said resistors, the resistor which is situated electrically more remotely from said phototube being shunted by a capacitor whereby to provide a time constant circuit, and a source of voltage which is substantially unafect ed by signalling currents generated by said phototube being connected to said time constant circuit at its terminal opposite said control voltage deriving point.

2. A signal reproducing system comprising means for generating signal currents in response to an incident modulated light beam, said means including an electron multiplier phototube, an output circuit for said phototube comprising a resistor, means for obtaining a control Voltage from a point on said resistor between its ends, means for utilizing said control voltage to control said phototube means for obtaining a signal output across said resistor, a portion of said resistor being shunted by a capacitor whereby to provide a time constant circuit, and a source of voltage which is substantially unaffected by signailing currents generated by said `phototube .being connected to, said time constant circuit at ,its terminal, opposite said control voltage deriving point.

3. Signal reproducing apparatus comprising an electron multiplier phototube, said phototube having, a plurality of electron multiplying dynodes, a Voltage divider for said dynodes com prising a plurality of series connected resistors,

said dynodes being connected to the junction points of said series combination of resistors, means for applying a voltage from a direct current source across said resistors, a cathode and anode for said phototube, said cathode being connected to the negative terminal of said direct current source, a connection from said anode to the lpositive terminal of said direct current source including two resistors connected in series, an output connection from said anode, a control voltage connection between said resistors, a connection from said control voltage connection to said means for applying a voltage, a capacitance shunting said resistor which is connected to the positive terminal of said power supply, said resistor and said capacitance providing a slowtimed R. C. load which responds only to slow variations in the anode current, a source of positive substantially constant voltage connected to said time -constant circuit at the end thereof which is more remote from said anode, said substantially constant voltage being of a value that it is nearly equal to the voltage drop in said load resistor.

4. Signal reproducing apparatus comprising an electron multiplier phototube, said phototube having electron multiplying means, means for applying a voltage from a direct current source to said electron multiplying means, a cathode and anode for said phototube, said cathode being connected to the negative terminal of said direct current source, a connection from said anode to the positive terminal of said direct current source including a resistor, an output connection from said anode, a control Voltage connection at a point on said resistor between its ends, a connection from said control voltage connection to said means for applying a voltage, a capacitance shunting a portion of said resistor, said resistor portion and said capacitance providing a slowtimed R. C. load which responds only to slow variations in the anode current, a source of positive substantially constant voltage connected to said time constant circuit at the end thereof which is most remote from said anode, said voltage being of a value that it is nearly equal to the voltage drop in the output portion of said resistor.

5. An automatic gain setting system comprising a generator of signalling currents having a sensitivity characteristic which varies with applied voltage, a power supply for said signalling current generator com-prising a high frequency oscillation generator and energy transfer means coupled to said generator, means to supply an operating voltage from said energy transfer r means to said signalling current generator, and

means responsive to the average value of the output of said signalling current generator for varying the output voltage of said high frequency oscillation generator.

6. An automatic gain setting system comprising a generator of signalling currents having a sensitivity characteristic which Varies with applied voltage, a direct current power supply source for said signalling current generator comprising a high frequency oscillation generator, said oscil- 

